Shape Stealer — ruling 1

This ability triggers once for each creature blocked by or blocking Shape Stealer. If multiple creatures block it, Shape Stealer's power and toughness will change for each one in succession. The first trigger put on the stack will be the last to resolve, so that will set Shape Stealer's final power and toughness.

Shape Stealer — ruling 2

The other creature's power and toughness are determined as the triggered ability resolves.

Shape Stealer — ruling 3

If the other creature has bushido, whether Shape Stealer gets the bushido bonus depends on whose turn it is. If Shape Stealer is blocking, it won't get the attacker's bushido bonus. If the Shape Stealer is being blocked, the bushido ability will resolve just before Shape Stealer determines what the creature's power and toughness are.

Shape Stealer — ruling 4

If Shape Stealer's new toughness is less than any damage on Shape Stealer, Shape Stealer will be destroyed as a state-based action once the triggered ability resolves.

Shape Stealer — ruling 5

Once the ability resolves, changing the other creature's power or toughness doesn't affect Shape Stealer.

Shape Stealer : règles, fonctionnement et interactions (MTG)

Shape Stealer is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever this creature blocks or becomes blocked by a creature, this creature's base power and toughness become equal to that creature's power and toughness until end of turn.

How it works : Shape Stealer

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2005· Wizards of the Coast
This ability triggers once for each creature blocked by or blocking Shape Stealer. If multiple creatures block it, Shape Stealer's power and toughness will change for each one in succession. The first trigger put on the stack will be the last to resolve, so that will set Shape Stealer's final power and toughness.
No other card is named in this ruling.
#2June 1, 2005· Wizards of the Coast
The other creature's power and toughness are determined as the triggered ability resolves.
No other card is named in this ruling.
#3June 1, 2005· Wizards of the Coast
If the other creature has bushido, whether Shape Stealer gets the bushido bonus depends on whose turn it is. If Shape Stealer is blocking, it won't get the attacker's bushido bonus. If the Shape Stealer is being blocked, the bushido ability will resolve just before Shape Stealer determines what the creature's power and toughness are.
No other card is named in this ruling.
#4June 1, 2005· Wizards of the Coast
If Shape Stealer's new toughness is less than any damage on Shape Stealer, Shape Stealer will be destroyed as a state-based action once the triggered ability resolves.
No other card is named in this ruling.
#5June 1, 2005· Wizards of the Coast
Once the ability resolves, changing the other creature's power or toughness doesn't affect Shape Stealer.
No other card is named in this ruling.